1. Field
This invention generally concerns robotic systems and is specifically concerned with an improved gripping mechanism for lightweight robotic systems for servicing heat exchanger tubes of a nuclear steam generator.
2. Related Art
In a pressurized water nuclear power electric generating system, the heat generated by the nuclear reaction is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a heat exchanger commonly referred to as a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. A transverse plate called a tubesheet, located at the lower end of the cylindrical section, divides the steam generator into a primary side, which is the lower hemispherical section below the tubesheet, and a secondary side above the tubesheet. A vertical wall bisects the primary side into an inlet section and an outlet section. The tubesheet is a thick carbon steel plate with an array of thousands of holes into which are inserted the ends of U-shaped tubes. One end of each U-shaped tube is inserted into a hole within the tubesheet which communicates with the inlet section of the primary side and the other end is inserted in a hole within the tubesheet which communicates with the outlet section. The primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tube and exits through the outlet section. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by heat given up by the primary coolant. Typically, there are thousands of small diameter U-shaped tubes which provide a large surface area for heat transfer. The number of tubes in a steam generator range from about 4,000 to 15,000. Some steam generators utilize straight length tubes each about 60 feet long. Most of the steam generators are constructed of U-shaped tubing or long vertical sections with two 90° bends joined by a shorter horizontal length tubing. During plant operation, the high pressure water that flows through the reactor core transports some amount of radioactive particles through the steam generators and some particles become deposited on the interior surface of the tubes. After plant operation, the steam generators become a source of radiation.
Occasionally, during the operation of the steam generator, degradation occurs in some of the tubes. This is undesirable because the primary coolant is radioactive and any leakage of the reactor coolant into the secondary side of the generator contaminates the steam. It is generally not practical, however, to replace degraded tubing, but instead the steam generator is periodically inspected and the affected tubes are plugged at both ends. In view of the thousands of tubes in the steam generator, plugging of a few tubes does not appreciably affect the efficiency of the heat transfer.
Because of the radiation hazard present in steam generators used in a nuclear power utility, the heat exchanger tubes of such steam generators must be, for the most part, remotely serviced to avoid exposing maintenance personnel to potentially harmful radiation. Consequently, a number of robotic systems have been developed for remotely performing repair and maintenance operations on these heat exchanger tubes. These robotic systems typically include some sort of robotic delivery arm in combination with any one of a number of specialized tools designed to be carried by the robotic arm, which are known in the art as “end effectors.” Some of the common robotic systems for this task utilize the holes in the tubesheet to anchor the robot via number of camlocks (typically four or more), for example, as shown in U.S. Pat. No. 7,314,343, assigned to the Assignee of this invention. Each charlock consists of a cylindrical arrangement of flexible “fingers” that protrude into a single tube and are expanded by a central cam actuated to engage the tube inner diameter surface. They thereby achieve anchoring from the resulting frictional force of the fingers on the tube inside diameter. This anchoring method is effective, but the problems are that the camlocks are costly, complex devices and they may release unexpectedly if the actuation force is lost.
Accordingly, it is an object of this invention to provide a simpler gripper capable of anchoring a robot to the underside of a tubesheet without the use of camlocks.
It is a further object of this invention to provide a single mechanism that provides both anchoring and rotational alignment.
It is an additional object of this invention to provide such a mechanism that supplies a very high gripping force through a mechanical advantage.
It is a further object of this invention to provide such a mechanism that automatically locks in place and requires no actuation force to stay locked.
It is a further object of this invention to provide such a mechanism that can release and re-grip very quickly.
It is an additional object of this invention to provide such a mechanism that is self-aligning and provides accurate locating.